The invention relates to a controller for controlling an actuator for a magnetic valve, and more specifically to a controller for an electromagnetic actuator for driving a valve of an engine mounted on such apparatus as an automobile and a boat.
Valve driving mechanism having an electromagnetic actuator has been known and called a magnetic valve. An electromagnetic actuator typically includes a moving iron or an armature which is placed between a pair of springs with given off-set load so that the armature positions at an intermediate part of a pair of electromagnets. A valve is connected to the armature. When electric power is supplied to the pair of electromagnets alternately, the armature is driven reciprocally in two opposite directions thereby driving the valve. Conventionally, the driving manner is as follows.
1) The magnetic attraction power that one of the electromagnets provides to the armature overcomes rebound power by the pair of springs and attracts the armature to make it seat on a seating position. The armature (valve) is released from the seating position by such a trigger as suspension of power supply to the electromagnet, and starts to displace in a cosine function manner by the force of the pair of springs.
2) At a timing according to the displacement of the armature, an appropriate current is supplied to the other electromagnet to produce magnetic flux which generates attraction force.
3) The magnetic flux rapidly grows as the armature approaches the other electromagnet that is producing the magnetic flux. The work by the attraction power generated by the other electromagnet overcomes the sum of (i) a small work by the residual magnetic flux produced by the one electromagnet which acts on the armature to pull it back and (ii) a mechanical loss which accounts for a large portion of the sum of work. Thus, the armature is attracted and seats on the other electromagnet.
4) At an appropriate timing as the armature seats, a constant current is supplied to the other electromagnet to hold the armature in the seated state.
In maintaining the armature in the seated state, it is desirable to supply the minimum driving current that can hold the armature in the seated state so as to minimize power consumption. However, when a minimum current is used every time the armature is to be held in the seated state, the armature moved to a seating position may from time to time leave the seating position due to secular changes of the electromagnetic actuator and/or variations of the movement. When the armature falls or lifts (collectively referred to as xe2x80x9cleavexe2x80x9d) from the seating position, such situation needs to be detected immediately and power supply needs to be boosted to pull the armature back to the seating position.
Conventionally, leaving of the armature was detected based on signals from a displacement sensor that detects displacement of the armature. Specifically, leaving (falling or lifting) of the armature is determined by detecting a situation that the sensor output does not indicate seated state of the armature in the period that the armature is in the seated state. In response to determination of leaving of the armature, a large current is supplied to the windings of the electromagnet to activate pullback operation immediately so that the armature may be pulled back to the seating position.
However, the conventional method includes the following problems. The air gap between the armature and the yoke of the electromagnet is very small when the armature is seated. The electromagnetic actuator has a very small magnetic reluctance when the armature is seated. When a constant current is supplied for holding the armature in the seated state, if the armature leaves the seating position by a small distance for some reasons, say less than 10 xcexcm from the seating position, the attraction force decreases. It is very difficult to detect such a small movement with the displacement sensor. For example, when the armature moves in the range of 7 mm in order to open and close a valve of an automobile engine, the displacement sensor can only detect the movement of the armature which is larger than {fraction (1/100)} of the moving range. That is, the sensor can only detect armature movement larger than 70 xcexcm due to noise and performance of the sensor. Leaving (falling or lifting) detection at 70 xcexcm point is too late to ensure pullback operation of the armature.
In addition, when pullback operation is activated at 70 xcexcm point, a larger current needs to be supplied, thereby increasing power consumption. This requires to increase the capacity of a driver element such as a field effect transistor, raising the cost of the driving circuit. Furthermore, a large current and the air gap produced by the leaving armature cause a large magnetic energy to be accumulated in the air gap. This magnetic energy is converted into kinetic energy of the armature and valve when the armature is attracted again to the seating position. As a result, seating speed of the armature becomes large producing a large collision sound when the armature seats.
As a specific example, a case for repetitively activating an electromagnetic actuator at a high speed as in the case of a valve train of an engine is described referring to FIG. 15. The left vertical axis shows the magnitude of displacement of the armature (mm) and current (A) supplied to the electromagnet. The right vertical axis shows attraction power (N) and voltage (V) applied to the electromagnet. As shown in the figures, the minimum attraction power (falling limit or leaving limit) that prevents the armature from leaving from the seating position is 485 N.
FIG. 15(a) shows a case in which the armature seats normally and a stable seated state is maintained. At time 0, the armature is released from one electromagnet and starts to move toward the other electromagnet by the operation of a pair of springs. During the period from time Te to Th, a constant voltage 42V is applied to the other electromagnet (over-excitation operation) to make the armature seat on the other electromagnet. After that, since the attraction force is a little larger than the leaving limit, a stable seated state is maintained. After the armature is seated, switching control of voltages 0 and +12V is performed to supply a constant holding current to the electromagnet.
FIG. 15(b) shows a case where a seated armature leaves the seating position. A displacement sensor detects the leaving movement of the armature when the armature reaches 70 xcexcm point, which is 1% of the lift (movement) range of 7 mm. A pullback operation is immediately initiated. The armature reaches 70 xcexcm point around time 6.33 ms. For 0.5 ms from time 6.33 ms, over-excitation voltage is applied. The voltage application period is determined according to the leaving extent (70 xcexcm).
After voltage application finished, a holding current value is renewed to a value which is larger than the preset normal holding current value by a predetermined value (for example, the predetermined value is 10% of the normal holding current value). Switching control of voltages of xc2x112V is carried out until the current converges into the renewed target holding current value. In the example shown in the figure, the switching control is carried out for 0.7 ms. Thereafter, switching control of voltages of +12V and 0V is performed so that current supplied to the electromagnet maintains the target holding current value.
In the example shown in FIG. 15(b), the armature leaves the seating position about 0.22 mm and is pulled back. The energy needed for the pullback is about 0.12 J. The seating speed (not shown) of the armature at pullback is approximately 0.6 m/s, which generates collision noise. Thus, activating pullback operation responsive to detection of the leaving armature by the displacement sensor causes delay in the pullback operation and requires a large energy for pullback. It produces a large seating speed leading to collision noise.
Thus, there is a need for a controller for an electromagnetic actuator which enables detection of a minute movement of the armature leaving the seating position and carries out pullback operation responsive to such detection.
According to one aspect of the invention, a controller is provided for controlling an electromagnetic actuator having a pair of springs acting on opposite directions, and an armature coupled to a mechanical element. The armature is connected to the springs and held in a neutral position given by the springs when the actuator is not activated. The actuator includes a pair of electromagnets for driving the armature between two end positions. The controller comprises current supplying means for supplying holding current to the electromagnet corresponding to one of the end positions when holding the armature in said one of the end positions. The controller includes means for determining that the armature is leaving the seated position when the holding current increases more than a predetermined value while the holding current is supplied to the electromagnet corresponding to said end position.
According to the invention, leaving armature is detected based on the variation of the holding current, which allows earlier detection of the leaving armature.
According to another aspect of the invention, the controller further comprises pullback means, responsive to determination of leaving of the armature, for applying voltage to the electromagnet corresponding to the end position, thereby pulling back the armature to the end position.
Because pullback operation is activated responsive to detection of leaving armature in terms of variation in the holding current, quick pullback is realized with relatively small energy.
According to further aspect of the invention, the current supplying means raises the holding current by a predetermined value. The holding current is supplied to the electromagnet corresponding to the end position after voltage is applied to the electromagnet by the pullback means.
Because the holding current is set to a relatively large value after the armature is pulled back from leaving, the armature will be prevented from leaving thereafter.
According to an aspect of the invention, the controller further includes setting means for setting the period for applying voltage to the electromagnet by said pullback means, in accordance with the difference between the time the armature leaves the seating position as determined by said determination means and a schedule release time of the armature. When the armature leaves, the period of the pullback operation can be controlled according to the timing of the release movement of the armature.
According to another aspect of the invention, the setting means shortens the period of voltage application to the electromagnet by said pullback means when the difference between the time the armature leaves the seating position as determined by said determination means and a scheduled release time of the armature is equal to or less than a predetermined value. Thus, delay of release operation of the armature is avoided.
According to yet another aspect of the invention, the controller includes a counter for counting the number of times the armature is held in the end position without leaving over a sequence of cycles. When the number of times shown by the counter is larger than a predetermined value, the supplying means decreases the holding current to supply to the electromagnet corresponding to said end position. Thus, optimization of the holding current for respective electromagnetic actuators can be realized.